The opening scene in “Good People” makes us squirm as ­often as we cheer for its heroine. That’s the beauty of playwright David Lindsay-Abaire’s characters; they are achingly imperfect, which makes them oh-so-compelling.

We meet Margaret (Johanna Day), a middle-aged, single mom from Southie, just as she’s about to lose her cashier’s job at the Dollar Store. She’s been called out to the smelly warehouse — beautifully rendered by Alexander Dodge to emphasize the impression that she is boxed in — by her manager ­Stevie (Nick Westrate), a kid whose mother went to school with Margaret. She knows where this conversation is going and in her desperation, plays every card she has to stall, deflect, guilt, or embarrass Stevie from firing her.